1) AOL instant messenger: The instant messenger AOL produces is one of the oldest and most popular. With over 200 million registered users AOL instant messaging has become one of the most popular ways for people to communicate online. The instant messenger AOL provides is free, and, not surprisingly, integrates seamlessly with other AOL software. AOL instant messaging software also provides security features like spyware scanning and a "who is" lookup (showing the user the registered owner of visited websites.)
2) MSN Messenger: Because of its integration into the Windows operating system, msn messenger is extremely common. When it was first released, msn allowed access to the AOL instant messaging network without the use of the instant messenger AOL provided (AIM). This option has since been removed and the software operates on the Microsoft .NET protocol, requiring a Microsoft passport to use. Msn messenger offers support for webcams, games, and voice conversations.
3) Yahoo! Messenger: Another popular messaging client, Yahoo! Messenger, operates on the Yahoo! protocol, and requires a Yahoo! Id. Many of its features are similar to the instant messenger AOL has as well as the Microsoft messenger, although it does allow users to customize the appearance of their messenger windows using what they call "IMVironments."
4) ICQ: ICQ was the first commonly available instant messaging program, released in 1996. It's been downloaded well over 300 million times, and uses a proprietary network protocol, though recently ICQ users have been able to communicate with the users of the instant messenger AOL provides (AIM). It is a feature rich application that included POP3 email support and a searchable user database. The most recent incarnation of the program is ICQ5, which attempts to attract younger users with features like games.
5) iChat: Developed by Apple, iChat is available on the Mac OS X platform. One of its benefits is that it allows communication with the instant messenger AOL provides (AIM). Unlike the situation when Microsoft first introduced its instant messenger, iChat is endorsed by AOL, which allows users to communicate beyond the relatively small community of Mac users. Apple has recently introduced iChat IV, which allows for voice and video communication amongst its users.
6) Google Talk: Google Talk is very new (announced on August 24, 2005) and faces stiff competition from the more established clients. The program is still in its infancy and lacks some of bells and whistles of other clients, but it does allow for voice communication. Google Talk also uses the Jabber protocol, so that users of other instant messaging clients may communicate with Google Talk users. The initial reviews of Google Talk have labeled it as lackluster, but that's likely to change with further development.
With the exception of Google Talk, which is still in the beta stages, these instant messaging clients all offer very similar features, and an average user's choice is more likely to be based on the operating system or internet provider (in the case of the AOL instant messaging client) than the features of the application itself.